Non-Kramers freezing and unfreezing of tunneling in the biaxial spin model

The ground state tunnel splitting for the biaxial spin model in the magnetic field, ${\cal H}=-D S_{x}^{2} + E S_{z}^{2} - g\mu_{\rm B}S_{z}H_{z}$, has been investigated using an instanton approach. We find a new type of spin instanton and a new quantum interference phenomenon associated with it: at a certain field, $H_{2}=2SE^{1/2}(D+E)^{1/2}/(g{\mu}_{\rm B})$, the dependence of the tunneling splitting on the field switches from oscillations to a monotonic growth. The predictions of the theory can be tested in Fe8 molecular nanomagnets